This invention relates in general to display drivers and, more particularly, to integrated circuits for driving light-emitting device displays.
Wireless communications devices typically include displays for conveying status and other information to a user of the wireless communications device. For example, a pager display can indicate that a page has been received or can display the phone number of the person paging. Most displays currently used in pagers are limited to displaying alphanumeric text because the display drivers only operate in a bilevel mode where pixels are either turned off or turned on to a fixed brightness level. Such display drivers cannot provide a variable pixel brightness needed for viewing images. Bilevel mode displays are adequate where the overall complexity of the pager is low, but with increasing functionality comes a need for a graphics user interface (GUI) to facilitate controlling the operation of the pagers. A high resolution, emissive display such as a light-emitting device such as a light-emitting diode (LED) display provides a GUI for viewing images, such as facsimile messages or images downloaded from the Internet, as well as alphanumeric characters. A typical LED display is organized into a plurality of rows and columns, and the display is operated by scanning, e.g., columns and activating rows to illuminate the pixels in the column.
A prior art display driver uses a binary counter combined with a decoder to select columns. The binary counter counts through the columns and the decoder selects a column based on the binary count. However, each column driver needs an extra latch at the outputs of the decoder to prevent the display from displaying random patterns during power up or system reset. The extra latch increases the cost and complexity of the display driver. The large number of pins needed for driving columns on larger displays further increases the cost. A less costly approach would be to use two smaller display drivers, one driving even columns from one end and another driving odd columns from the other end of the display. However, prior art display drivers only scan columns in one direction, say from left to right, so they can only drive the display from one specific end. If they are connected to the opposite end, the columns will be scanned backwards. As a result, cost is increased because either a complex interconnect scheme or different versions of the display driver are needed for driving the display from both ends.
For activating pixels, row drivers provide gray scale shading when displaying graphics images and pixel on/off control when displaying text. Gray scale shading requires more data transfers and more complex circuits because more data bits are needed for graphics than for text. Displaying graphics therefore requires higher frequency data transfers to accommodate the increased data flow, which increases power consumption and reduces the time between battery recharges.
Hence there is a need for a graphics mode display driver which can drive either end of a display while reducing power consumption in applications such as portable wireless communications devices.